1. Field of the Invention
This invention relates to a sun sensor for detecting an angle of incidence of sunlight. More specifically, the invention relates to a sun sensor which is used for determining and controlling the attitude of a satellite.
2. Description of Related Art
A spacecraft, such as a satellite, needs to stabilize its attitude so that its solar cells can receive solar energy or so that an antenna can be oriented to transmit or receive signals to or from a defined direction on earth. To accomplish such orientation, a sun sensor can be employed to detect an angle of incidence of the sunlight.
FIG. 11 shows a known sun sensor for detecting an angle of incidence of the sunlight disclosed in U.S. patent application 07/023,719. The sensor includes a shielding surface 11 having a light transmitting slit 12, which is long and slender, a photodetector 13 and a processor 15. Photodetector 13 has plural photodiodes 14, for example, arranged along a line perpendicular to slit 12. Photodiodes 14 generate electrical signals corresponding to the intensity of the sunlight which is received by photodiodes 14. Processor 15 is provided with the electrical signals and computes an angle of incidence of the sunlight. When the sunlight passes through slit 12, the sunlight is diffracted. Therefore, the sunlight reaching photodetector 13 has the intensity distribution shown in FIG. 12(a). Processor 15 quantizes the electrical signals and converts the electrical signals into digital signals as shown in FIG. 12(b). Also, the digital signals are converted into binary signals "1" and "0" in comparison with a reference level as shown in FIG. 12(c). A center position x.sub.c of the binary signals is calculated from positions x.sub.1 and x.sub.2 where the value of the binary signal changes. Thus, a center position on photodetector 13, corresponding to the center position of the intensity distribution of the sunlight, is determined. The angle of incidence of the sunlight is highly accurately derived from the center position on photodetector 13.
In a spacecraft, such as a satellite, angular information with respect to two axes, perpendicular to each other are necessary to control attitude. Therefore, as shown in FIG. 13, two sun sensors, having slits perpendicular each other, are used for detecting the solar angular information with respect to the two axes. The known device is large because the sensor has to employ two photodetectors. The limited space on a spacecraft, and the attendant cost of such space, makes such a known sensor undesirable.